On Fri, Jan 20, 2023 at 02:37:19PM -0500, Jason Merrill wrote:
> On 1/19/23 21:03, Marek Polacek wrote:
> > We shouldn't emit "right operand of comma operator has no effect"
> > when that comma operator was created by the compiler for "new int{}".
> > convert_to_void/COMPOUND_EXPR already checks warning_suppressed_p so
> > we can just suppress -Wunused-value.
> >
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > PR c++/107797
> >
> > gcc/cp/ChangeLog:
> >
> > * cvt.cc (ocp_convert): copy_warning when creating a new
> > COMPOUND_EXPR.
> > * init.cc (build_new_1): Suppress -Wunused-value on
> > compiler-generated COMPOUND_EXPRs.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/warn/Wunused-value-1.C: New test.
> > ---
> > gcc/cp/cvt.cc | 6 ++++--
> > gcc/cp/init.cc | 2 ++
> > gcc/testsuite/g++.dg/warn/Wunused-value-1.C | 12 ++++++++++++
> > 3 files changed, 18 insertions(+), 2 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/warn/Wunused-value-1.C
> >
> > diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> > index f816c474cef..52e96fbe590 100644
> > --- a/gcc/cp/init.cc
> > +++ b/gcc/cp/init.cc
> > @@ -3800,6 +3800,8 @@ build_new_1 (vec<tree, va_gc> **placement, tree type,
> > tree nelts,
> > if (cookie_expr)
> > rval = build2 (COMPOUND_EXPR, TREE_TYPE (rval), cookie_expr, rval);
> > + suppress_warning (rval, OPT_Wunused_value);
>
> This makes sense, but IIUC since rval is built with no location, this just
> sets nowarning_flag?
Correct (I didn't realize this when writing the patch). There's another
suppress_warning on a location-less expr just a few lines above.
> > if (rval == data_addr && TREE_CODE (alloc_expr) == TARGET_EXPR)
> > /* If we don't have an initializer or a cookie, strip the TARGET_EXPR
> > and return the call (which doesn't need to be adjusted). */
> > diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
> > index 0cbfd8060cb..17827d06a4a 100644
> > --- a/gcc/cp/cvt.cc
> > +++ b/gcc/cp/cvt.cc
> > @@ -711,8 +711,10 @@ ocp_convert (tree type, tree expr, int convtype, int
> > flags,
> > return error_mark_node;
> > if (e == TREE_OPERAND (expr, 1))
> > return expr;
> > - return build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE
> > (e),
> > - TREE_OPERAND (expr, 0), e);
> > + e = build2_loc (EXPR_LOCATION (expr), COMPOUND_EXPR, TREE_TYPE (e),
> > + TREE_OPERAND (expr, 0), e);
> > + copy_warning (e, expr);
>
> And so I don't know what effect this would have; copy_warning doesn't seem
> to propagate nowarning_flag, which seems like a bug.
It actually does propagate nowarning_flag: in copy_warning we have
219 /* The no-warning bit might be set even if the map has not been
consulted, or
220 otherwise if there's no entry in the map. */
221 set_no_warning_bit (to, supp);
so 'e' ends up with TREE_NO_WARNING set.
I would not think we want to build 'rval' in build_new_1 with a location
or use TREE_NO_WARNING directly instead of suppress_warning so I'm leaving
the patch as-is, but please let me know if you disagree.
Marek
